1. Field of the Invention
The present invention relates to stable pigment dispersions and pigmented inkjet inks including color pigments that are stabilized by polymeric dispersants in a non-aqueous medium using dispersion synergists which exhibit a structural similarity with the color pigments.
2. Description of the Related Art
Pigment dispersions are made using a dispersant. A dispersant is a substance for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. Dispersants are generally surface-active materials having an anionic, cationic or non-ionic structure. The presence of a dispersant substantially reduces the required dispersing energy. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersing operation, due to mutual attraction forces. The use of dispersants also counteracts this re-agglomeration tendency of the pigment particles.
The dispersant has to meet particularly high requirements when used for inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of the pigment particles through the nozzles of the print head, which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided during the standby periods of the printer.
Polymeric dispersants contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains compatible with the dispersion medium, thus stabilizing the pigment particles in the dispersion medium. Typical polymeric dispersants are graft copolymers and block copolymers.
In aqueous inkjet inks, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium.
The preparation of good thermally stable dispersions with submicron particles is more difficult for non-aqueous inkjet inks, such as solvent based, oil based and radiation curable inkjet inks. The pigments are especially difficult to disperse when they have a non-polar surface.
These problems have led to the design of very specific polymeric dispersants wherein the anchor groups are pigment derivatives. For example, EP 0763378 A (TOYO INK) discloses a pigment composition including a non-aqueous type pigment dispersing agent having a portion which has a high affinity with a pigment and which has at least one type selected from the group consisting of an organic dye, anthraquinone and acridone only at a terminal end or at both terminal ends of at least one polymer selected from a linear urethane polymer and a linear acrylic polymer, and a pigment.
Another approach for dispersing pigments with non-polar surfaces in non-aqueous dispersion media is changing the surface to a more polar surface by addition of compounds known as dispersion synergists. A dispersion synergist is a compound that promotes the adsorption of the polymeric dispersant on the surface of the pigment. It is suggested that the synergist should have the chemical structure of the pigment including one or more sulphonic acid groups or ammonium salts thereof.
U.S. Pat. No. 4,461,647 (ICI) discloses a dispersion of a pigment in an organic liquid containing a water-insoluble asymmetric disazo compound including a central divalent group free from acidic and other ionic substituents linked through azo groups to two monovalent end groups wherein one end group, the first, is free from acidic and other ionic substituents and the other end group, the second, carries a single substituted ammonium-acid salt group.
U.S. Pat. No. 4,057,436 (ICI) discloses pigment dispersions in organic liquids using polymeric or resinous dispersing agents in the presence of a substituted ammonium salt of a colored acid wherein there are between 16 and 60 carbon atoms contained in at least 3 chains attached to the N-atom of the substituted ammonium ion.
U.S. Pat. No. 6,641,655 (AVECIA) discloses the use of a diquaternary ammonium salt of a colored acid as fluidizing agent wherein the diquaternary ammonium cation contains two or more nitrogen atoms.
According to the best of our knowledge, only two products have been commercialized, i.e., C.I. Pigment Blue 15 and C.I. Pigment Yellow 12 both substituted by sulphonic acid groups. Although these dispersion synergists work fine for some pigments, many other pigments cannot be dispersed to an acceptable quality in a non-aqueous medium.
EP 1146088 A (TOYO INK) discloses a method of producing a disazo pigment, in which a coupling reaction is carried out while feeding, into an acidic aqueous solution, a tetrazo aqueous solution containing a tetrazo component of benzidines and a coupler aqueous solution containing a coupling component. Aqueous printing inks using the wet pigment compositions are also disclosed.
EP 957136 A (DAINIPPON INK) discloses additives including carboxylic acid functions for a disazo pigment with superior dispersibility, suitable for printing inks.
U.S. Pat. No. 3,759,731 (HOECHST) discloses mixed crystals of polar and nonpolar disazo dyes combined with an oil-modified alkyd resin.
GB 2356634 (ILFORD) discloses mono-azo compounds derived from diazotized 2-alkoxycarbonylanilines and N-[(carboxy/sulpho)-phenyl]acetoacetamides and their use in aqueous ink compositions.
There is a need for dispersing other pigments since each pigment has a different set of properties. Pigments are selected for inkjet inks based upon the properties required for their application, for example, for outdoor inkjet applications light stability may be more important than producing inkjet images with high color strength.
For consistent image quality, inkjet inks require a dispersion stability capable of dealing with high temperatures (above 60° C.) during transport of the ink to a customer, jetting at elevated temperatures and changes in the dispersion medium of the inkjet ink during use, for example, evaporation of solvent and increasing concentrations of humectants, penetrants and other additives.
Therefore, it is highly desirable to be able to manufacture stable pigmented inkjet inks using dispersion synergists in a non-aqueous medium for a wide range of color pigments.